Offset printing presses operate under the principle that water and oil do not mix. A flexible plate containing an image to be printed is mounted on a drum also referred to as a press or plate cylinder. FIGS. 1 and 2 show a simplified view of rollers used to apply "water" and "ink" to an "image" plate mounted on a plate cylinder or drum 11. A first series of ink rollers 22 to 30 are coupled between an ink reservoir 21 and the plate cylinder 11 and are designed to make contact with the plate 11, with the end rollers (e.g., 29, 30) distributing ink to the imaged areas. A second series of rollers 32 to 35 are coupled between a reservoir 31 containing a water solution and the plate cylinder 11. The last roller 35, also denoted as a "water form roller", makes contact with plate cylinder 11, distributing a thin film of water or chemical solution (fountain solution) to the non-image areas of plate 11. This "dampening" action ( i.e., the application of a water solution to the plate) prevents the ink ("oil") from mixing or adhering to the non-image areas of the flexible image plate.
Traditionally one or more of the dampening rollers are covered with a paper or fabric material having absorbent properties to allow for the rapid and even transfer of water from the fountain or pan to the printing plate. The construction of the cover may be of cotton, rayon, or any number of other fabrics or paper substrates capable of wicking fountain solutions. This dampening material has a limited useful life that is measured in weeks, days, or even hours depending on a number of operational factors. Most commonly, the fabric or paper is eventually worn out by contact with the printing plate or becomes impregnated with ink solids which interfere with the balanced transfer of fountain solution. Soiled, contaminated, or exhausted covers must be manually stripped from the roller and replaced.
Conventionally, it has been the practice to remove a worn out dampening cover from a roller by cutting it with a sharp knife or blade, a practice which often results in damage to the roller, while risking the safety of the individual doing the cutting. Mounting a new cover on the roller is a time consuming task that involves forcing the cover over the roller surface. If the new cover has not been positioned properly so that it is centered on the roller, it must be removed and the process repeated until the distribution of the material is balanced. Because a taut fit around the circumference is essential to the proper rotation and operation of the roller, the mounting of new covers is an arduous task, requiring substantial physical effort to overcome the resistance of the closely matched outside diameter of the roller and the inside diameter of the dampening material.
The ends of the material must be secured to the roller ends to prevent any lateral movement. This is most commonly achieved through the use of drawstrings if so provided by the manufacturer of the dampening cover. These draw strings must be pulled tight over the two lateral ends of the roller and tied in a secure knot. If such retention devices are not provided, the user must carefully sew the cover ends using a needle and thread.
Some dampening covers have a shrinkable yarn imbedded in the construction of the material which is activated upon being wetted with water. This shrinkage is intended to provide a tight and secure fit through circumferential compression, eliminating the need for drawstrings or sewing. Due to the torque and high speeds encountered during the rotation of the dampening roller, this constriction mounting method often fails to provide adequate adhesion to the roller, resulting in the cover "walking" or sliding off the roller. Also, the shrinkage is very often irregular, resulting in an uneven dampening surface and impaired water transfer.
Dampening rollers are also often subject to contact with mechanical linkages, such as drive collars and/or gears, which often shear the cover ends and cut the drawstrings, stitching, or other containment devices. This causes the cover to break free and move side to side, resulting in a loss of the dampening action and the need for immediate replacement.
Other factors often necessitate the removal and replacement of dampening covers. The rubber rollers used to support them often swell over time due to absorption of chemicals. This expansion most often takes place at the roller ends where caustic compounds found in inks, solvents, and dampening solutions can easily permeate the exposed rubber. Once swelled, the roller is no longer of consistent diameter and can not provide even contact with the image plate. Unbalanced dampening action results and the dampening cover must be removed so the roller can be reground to uniform thickness or replaced.
Regardless of the method used to mount and secure the dampening material, it is a laborious and time consuming task which is most often performed by the pressperson, a highly paid technical position. The result is that the cost to change a cover is often substantially more than the cost of the material itself. The traditional cover changing process may require 15 to 30 minutes on average.
It is an object of the invention to provide a unitary disposable component comprising the integral combination of a dampening roller and cover which can be easily and rapidly replaced (e.g., in less than 30 seconds) and which can be manufactured economically.
The physical composition of dampening rollers has been limited to natural or synthetic rubber in order to provide a resilient mounting surface for the dampening cover. The durometer reading, a measure of a rubber surface's resiliency, is typically specified as a value between 23 and 29 for dampening rollers. This pliant rating provides the shock absorbent cushioning or "give" required to maintain even and consistent plate-to-dampening cover contact. Inconsistencies in the thickness of the dampening material as well as plus or minus variances in the concentric trueness of the roller itself require the compensatory spring action of a resilient material such as rubber. The yielding properties of the rubber also absorb bounce or vibration encountered during high speed rotation.
In consideration of the inconsistencies in the thickness that result during the manufacturing of covers and rollers, it should be noted that in actual use the pressure setting applied between the dampening cover (e.g., assembly 35 in FIGS. 1 and 2, herein) and the plate 11 is intentionally increased beyond the point of initial contact as compensation. This exaggerated pressure setting also provides a slight squeegee effect assuring the complete release of the fountain solution onto the plate.
Consideration must also be given to the fact that as the dampening roller rotates over the surface of the plate, it passes over a gap or indentation 13 in the supportive plate cylinder. This gap 13, which normally is about 2 to 4 inches in width, is a necessary indent which provides a point of attachment for the head and tail of the flexible image plate. As the dampening roller passes over the beginning of gap 13, contact with the cylinder and plate is momentarily lost. As the cylinder and roller continue to rotate contact is once again made with the cylinder and plate surface. It is at this point that the roller/cover assembly is subjected to sudden impact and stress as the resumed contact compresses the dampening cover/roller assembly. The shock absorption provided by the rubber construction of the roller attenuates and dissipates the impact which would normally be transferred to the bearings, shaft, journal, or other supportive structures, causing wear and/or damage to the dampening roller and eventually to the press itself.
In a former method, as described in U.S. Pat. No. 2,966,724, the subject matter of which is incorporated herein by reference, a disposable roller comprised of a thick core is also prefitted with a dampening covering. This concept calls for the addition of metal or plastic bearings to provide precise support and rotation of the roller. The thick, heavy, construction of the tube adds to the weight and cost of the roller and the use of bearing assemblies also adds substantially to the cost and complexity of the system, negating any claim as a low cost, disposable concept. Additionally, there is no cushioning effect provided by the design, as the dampening material is adhered directly to a non-resilient solid roller.
Another approach is to be found in U.S. Pat. No. 3,919,754, the subject matter of which is incorporated herein by reference, in which a multiple component premounted roller/dampening cover assembly is introduced. FIG. 6 of the 3,919,754 patent shows a three part disposable cover comprising an inner core, a surrounding tube of a resilient material, and an outer sleeve of water absorbent material. In use, the three part cover assembly is mounted on a supportive roller (i.e., item 10 shown in FIG. 2 of the '754 patent) of the printing press and held in place by removable, machined end caps containing inset metal bearing support assemblies. The patented design necessitates additional down time during cover changes, as the end caps must be removed and then replaced each time a premounted sleeve is changed. The use of metal machined parts adds substantially to the cost and complexity of the system. Additionally, the bearings are subject to wear requiring replacement as in a traditional roller and any mishandling of the supportive metal components which might result in a burr, dent, or abrasion could necessitate replacement.
It is, therefore, an object of the present invention to provide a unitary disposable component comprising an integral dampening roller/dampening cover assembly that is simple in construction, low in cost, light in weight, and which can be quickly and easily mounted and removed as necessary.
It is another object of the present invention to provide a stable and precise support for a dampening roller assembly without the need for conventional bearings, bushings, or other supportive devices as required in previous designs.
It is still a further object of the present invention to allow for variation in the formulation, construction, and thickness of different dampening materials so that the cover assembly is not restricted to a singular brand, physical composition, or material type.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is understood, however, that the drawings are designed for the purposes of illustration only and not as a definition of the limits of the invention, for which reference should be made to the appending claims.